1. Field of the Invention
The present invention relates to a changeover circuit for a retrace capacitor for generating retrace pulses which are necessary for electron beam scanning when a picture is displayed on the screen of a display monitor.
2. Description of the Related Art
In a home TV cathode-ray tube, the electric signals obtained by scanning a picture by an imaging device are scanned on a monitor screen by an electron beam in the same way as by the imaging device so as to display the picture.
A display monitor has recently been used as a displaying device for information processing such as a computer and a word processor and it is capable of displaying a picture of higher resolution than a TV picture tube.
In this type of device in which an electron beam is deflected by a deflection circuit for the purpose of horizontal and vertical scanning on the monitor screen, it is necessary to generate retrace pulses which correspond to a predetermined scanning frequency. For this purpose, a changeover circuit for a retrace capacitor having a structure such as that shown in FIG. 2 is conventionally used.
FIG. 2 shows a changeover circuit for a retrace capacitor of a monitor FA3415 produced by Mitsubishi Electric Corporation.
As shown in FIG. 2, two retrace capacitors 2, 3 are connected in parallel through, the supply terminals on the side closer to the live potential 1, and field-effect transistors (hereinunder referred to as "FET's") 4, 5 for executing a switching operation are inserted between the other terminals of the retrace capacitors 2 and 3, respectively, and ground 6. An output signal having a predetermined level is output from a control circuit 7 to the gates of the FET's 4 and 5. The control circuit 7 controls the ON-OFF switching operations of the FET's 4 and 5 in accordance with a change in the output level.
A conventional changeover circuit for a retrace capacitor having the above-described structure generates retrace pulses for electron beam scanning by switching the FET's 4 and 5 by the control circuit 7 in accordance with a predetermined frequency pulse.
In the conventional changeover circuit for retrace capacitors, since a voltage as high as about 1,000 V is applied to both ends of the retrace capacitors 2 and 3 shown in FIG. 2, the FET's 4 and 5 used for switching the retrace capacitors also must have a withstand voltage of about 1,000 V.
Each of the FET's 4, 5 has an on-state resistance (a resistance which is existent when the FET is switched on), which generates heat when the FET is switched on. To prevent the generation of heat, a heat sink for cooling the FET's 4, 5 is necessary.
In this way, in the case of using an FET, the changeover circuit inconveniently cost a great deal.
If a relay is used for the switching operation in place of the FET, it is possible to reduce the cost.
However, since the on-state resistance of the relay is extremely small and the inrush current becomes very large, it is necessary to use a relay having a large rated current, which leads to the increase in size of the changeover circuit.